Update public documentation with juju 3.4

[DnPlas]

Context

Based on the discussion in #940, we want to make sure the public documentation for Charmed Kubeflow shows the right version of juju to use.

What needs to get done

1. Go into charmed-kubeflow.io and check for places where juju 3 is mentioned
2. Make sure each entry mentions the support for juju 3.4 for CKF 1.8
3. Remove any traces of juju 3.1 and/or 3.5

Definition of Done

Public documentation in charmed-kubeflow.io does not make any reference to an unsupported version of juju.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5954.

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[DnPlas]

The most important guides that should be updated are:

• Upgrade 1.7 -> 1.8
• Get started